Present day commercially available boron nitride having a cubic structure at room temperature generally has been prepared by using high pressure technology and processing techniques. Cubic boron nitride (CBN) produced by this process is expensive because of the complexity and attendant low volume associated with such high pressure processing.
Cutting tools coated with CBN are particularly attractive because higher cutting rates, compared to tungsten carbide, are possible. Also because a coolant can be used with CBN cutting tools, better surface finishes usually can be obtained than when using the Al.sub.2 O.sub.3 -TiC-TiN-TiO.sub.2 type cutting tools dry.
U.S. Pat. No. 3,918,219, Wentorf, Jr., et al., discloses the conversion of CBN from hexagonal boron nitride (HBN) using certain aluminum alloys as catalysts. This process is carried on at a high pressure and high temperature. CBN can be formed from BN at temperatures near 1800.degree. C. and at pressures of approximately 85 kilobars with various materials such as alkali and alkaline earth metals, and aluminide forming materials as catalysts.
U.S. Pat. No. 3,791,852, Bunshah, describes a process and apparatus for the production of carbide films by physical vapor deposition by means of an activated reactive evaporation deposition technique. This patent, including its drawing figure of a typical apparatus and description thereof, is incorporated in this disclosure by reference.
U.S. Pat. No. 4,297,387, Beale, discloses a process of making CBN utilizing an activated reactive evaporation technique which involves evaporating an alloy of boron and aluminum and at least one of cobalt, nickel, manganese, or other aluminide-forming elements in the presence of ammonia gas while generating an electrical field in the zone for ionizing the metal vapors and gas atoms in the zone. The temperature is maintained at least at 300.degree. C. with a preferred temperature of between 500.degree. C. and 1100.degree. C. with the ammonia gas pressure preferably between 1.times.10.sup.-4 torr to 8.times.10.sup.-3 torr.
U.S. Pat. No. 4,297,387, Beale, relies on the vaporization of a cubic phase nucleator which also functions as a barrier to possible dislocation motion or transformation nuclei formation in the form of an aluminide made from cobalt, nickel, manganese, zirconium, or iron.
This patent also utilizes ammonia (NH.sub.3) gas which is hygroscopic. Because it is hygroscopic, it presents difficulties with respect to the introduction of impurities.
In applicant's copending patent application filed on the same date as this patent application, there is disclosed a process involving the formation of CBN at relatively low temperatures utilizing the activated reactive evaporation deposition process without the need to include aluminum as an alloying element in the metal alloy source so as to form an aluminide. However, this patent application utilizes ammonia gas as a source of nitrogen.